Dissociable Contributions of Left and Right Dorsolateral Prefrontal Cortex in Planning

Abstract

It is well established that the mid-dorsolateral prefrontal cortex (dlPFC) plays a critical role in planning. Neuroimaging studies have yielded predominantly bilateral dlPFC activations, but the existence and nature of functionally specific contributions of left and right dlPFC have remained elusive. In recent experiments, 2 independent parameters have been identified which substantially determine planning: 1) the degree of interdependence between consecutive steps (search depth) and 2) the degree to which the configuration of the goal state renders the order of single steps either clearly evident or ambiguous (goal hierarchy). Thus, search depth affects the actual mental generation and evaluation of action sequences, whereas goal hierarchy reflects the extraction of goal information from an encountered problem. Here, both parameters were independently manipulated in an event-related functional magnetic resonance imaging study using the Tower of London task. Results revealed a double dissociation as indicated by a significant crossover interaction of hemisphere and task parameter: in left dlPFC, activations were stronger for higher demands on goal hierarchy than on search depth, whereas the reversed result emerged in right dlPFC. In conclusion, often observed bilateral patterns of dlPFC activation in complex tasks may reflect the concomitant operation of specific cognitive processes that show opposing lateralizations.

Set-shifting as a component process of goal-directed problem-solving

“…This supports the conclusion that while problems may vary in difficulty, the differences in the problems considered here do not reflect a critical dimension when it comes to differentially engaging specific executive functions. Kaller et al (2011b) came to a similar conclusion in motivating their imaging study of performance on a variant of the ToL. They argued that the critical dimensions were not the number of moves required or the presence of superficially tempting moves per se, but subgoal order ambiguity and the need for intermediate moves.…”

“…This is not the case for "flat" target states, where one disk is on each peg, and where the order in which subgoals must be achieved so as to minimise total moves is ambiguous. The tower/flat distinction applies equally to the ToL as it does to the ToH (Waldau, 1999;Kaller et al, 2008;Kaller et al, 2011a), and as noted in the introduction, Kaller et al (2011b) found that problems with ambiguous subgoal ordering engage left lateral PFC relatively more than those without (while problems that require intermediate moves activated right lateral PFC more than those that did not). Ambiguous subgoal ordering again implies the need for a switching function: successful solution of such problems will require considering different orderings of the subgoals, and hence switching between subgoals when planning moves.…”

“…Berg & Byrd, 2002). This method has been used in previous ToL research to present multiple ToL problems to participants while controlling for problem structure (e.g., Kaller et al, 2008Kaller et al, , 2011b. The order of presentation of the six problems was also randomised within each block.…”

“…Consistent with such a strategy, problems where the goal state involves all three balls on the same peg (i.e., problems with an unambiguous subgoal ordering) are more readily solved than those in which the goal state involves all three balls on different pegs (i.e., where the order in which subgoals should be achieved is ambiguous: Kaller et al, 2008;Klahr & Robinson, 1981;Newman & Pittman, 2007;Waldau, 1999). Both factors were found by Kaller et al (2011b) to increase dorsolateral prefrontal cortical activity (DLPFC, as measured by BOLD), but ambiguous subgoal structure was found to differentially recruit left DLPFC while the need for intermediate moves was found to differentially recruit right DLPFC. While this study is suggestive of different cognitive processes being involved in different problems, it is difficult to identify the specific processes without the development of a complete process model of ToL performance and its application to the different types of problem.…”

“…In one recent study, Kaller et al (2011b) explored the effect of two specific factors relating to different ToL problemswhether the goal state unambiguously determines the order of subgoals and whether intermediate moves were required in order to solve a ToL problem -on the neural correlates of ToL solution. With regard to the former factor, previous research has shown that people typically solve Tower problems by focussing on one disk or ball at a time -getting that disk/ball into its correct target position before moving on to the next disk/ball .…”